The studies described in this proposal are directed at investigating the control strategies adopted by the CNS to execute visually evoked arm postures and movements in monkeys, humans and patients with neurological disorders. With respect to visually evoked postures, our approach is to evaluate the stiffness at the hand when the arm is positioned under a visual target. The net stiffness at the hand represents the interactions of all of the muscles acting on the arm and subsumes the effects of volitional and reflex mechanisms. We will determine the stiffness obtained by displacing the hand in many different directions, each time determining the resultant elastic restoring forces. In this way, we will be able to describe the shape, orientation and magnitude of the stiffness field at the hand for a number of positions in the arm work-space. The stiffness field provides a rich description of arm posture and a measure of the control exerted by the CNS upon the muscle spring-like properties. To understand the nature of this control, we plan to study the field under a variety of experimental situations. At the conclusion of these experiments we will have not only a precise difinition of arm posture, but also insights into the relationship between posture and movement. With respect to visually evoked movements, we will study the kinematic features in the case of three-joint planar arm movements to targets in normal humans and monkeys. In these studies we will search for kinematic invariances in terms of hand trajectory and tangential velocity. We will also investigate the way in which our studies relate to CNS structures such as the cerebellum by performing kinematic and postural studies in patients with cerebellar disorders and in monkeys with lesions in areas corresponding to the human pathology. Finally, theoretical and simulation studies will complement and aid the interpretation of the experimental results. We will formally evaluate the stiffness field to determine the magnitude of the elastic and non-elastic components. In addition, with computer simulation, we will study the interplay of the elastic, viscous and inertial effects of the musculo-skeletal system on arm posture and movement.